1. Field of the Invention
The present invention relates to a technology for detecting acceleration based on an oscillating frequency using a piezoelectric plate such as a crystal plate.
2. Description of Related Art
In order to measure an earthquake or the like, one important issue is detecting weak and low frequency acceleration. It is desirable to measure with high accuracy using a structure that is as simple as possible when carrying out this kind of measurement. As a sensor for detecting weak and low frequency acceleration, a servo-type acceleration measuring apparatus is often used.
In general, a servo-type acceleration measuring apparatus is constituted by a pendulum, a pendulum position detector, an actuator that applies a force to the pendulum, and a regulating unit that controls the actuator based on a detection result by the pendulum position detector. The pendulum is constituted by a spindle and a spring. One end of the spring is fixed to a container of the acceleration measuring apparatus, and the position of the spindle is displaced relative to the container by the action of an inertial force when acceleration is exerted on the acceleration measuring apparatus. A resonance frequency of the pendulum is set extremely low, and even a slight acceleration largely displaces the pendulum. The displacement of the pendulum relative to the container is proportional to the exerted acceleration in a frequency range that is lower than the resonance frequency of the pendulum. The pendulum position detector is a sensor that detects positional displacement of the pendulum relative to the container. The actuator consists of a coil provided to the pendulum and a magnetic circuit provided to the container. The position of the pendulum can be displaced by an electromagnetic force that is generated when a current is applied to the coil. The regulating unit is for applying a current to the coil of the actuator based on positional displacement data of the pendulum obtained by the pendulum position detector.
If acceleration from an outside force is exerted on the acceleration measuring apparatus, the position of the pendulum is displaced by an inertial force. At this time, a current is applied to the actuator from the regulating unit, and by exerting an electromagnetic force that is the same size as the inertial force but is in the opposite direction on the pendulum, the pendulum can be maintained in a stationary state. Therefore, by detecting the positional displacement of the pendulum with the pendulum position detector and then operating the actuator so that the positional displacement is zero, the positional displacement of the pendulum can be feedback controlled. By measuring the output of the actuator at this time, for example by measuring the current value applied to the coil, the acceleration of the outside force can be measured. This kind of servo-type acceleration measuring apparatus has characteristics of high accuracy and high resolution and is capable of measuring frequencies of about 0 to 400 Hz.
As a method for detecting the position of the pendulum in a servo mechanism of a servo-type acceleration measuring apparatus, an optical method and a capacitor method are mainly used. Among these, an optical-type pendulum position detector uses a laser diode, a two-element segmented photodiode, and a lens. As a method for detection using an optical-type pendulum position detector, a type of differential method is employed. However, an optical-type pendulum position detector presents problems in that it has a complex structure and the life of the photodiode is short. A capacitor-type pendulum position detector has a structure in which a capacitance is formed such that it changes by the positional displacement of the pendulum, and the pendulum position is detected by detecting the change in capacitance. However, the influence of noise cannot be eliminated, and thus it is difficult to detect the acceleration with high accuracy.
Patent Document 1 discloses an acceleration sensor in which a constant current is made to flow through a movable electrode, and acceleration is detected by detecting the number of pulses of an induced current generated in a fixed electrode that opposes the movable electrode. However, this acceleration sensor is different from the present invention. Patent Document 2 discloses a capacitance change detection-type acceleration sensor, in which two variable capacitances are formed between a movable center plate and fixed plates provided on both sides of the movable center plate, and antiphase pulse voltages are respectively applied to the two fixed plates. Both variable capacitances change when the position of the center plate is displaced due to the generation of acceleration, and the acceleration is detected at this time by detecting a shift in the pulse phase of the voltage applied from the fixed plates to the center plate. However, this capacitance change detection-type acceleration sensor is different from the present invention.    [Patent Document 1] Japanese Patent Application Laid-Open (JP-A) No. H7-167885    [Patent Document 2] Japanese Patent Application Laid-Open (JP-A) No. 2004-198310